My first reaction was that this was M. sheldonii, but not microcarpa. My image of microcarpa is of more radial spines and finer spines that here, and maybe lighter central spines. M. sheldonii usually has heavier spines, and fewer radials.
But then I wondered if this could perhaps be M. mazatlanensis rather than M. sheldonii. They are similar, and equally variable, but normally mazatlanensis has slimmer stems and is more heavily branched. The flowers also tend to be of one uniform colour rather than shading from light to pale.
I do find it is quite difficult to be sure, as both species are quite variable, and maybe we need to wait until the plant is a bit older and one can see how it develops as far as offsets and branches are concerned.
Sorry I can't be more defintive - maybe others can shed more light.